The development of high-power microwave sources has proceeded slowly over several decades, motivated by different applications at different times. Immediately after World War II, for example, tubes which had been developed for radar and for high-power transmitters were needed to power high-energy particle accelerators. The most dramatic development took place at Stanford University. It was there that the klystron was rapidly developed from the kilowatt level to peak powers exceeding a megawatt. After further development the klystron rapidly became the accepted power tube for a large number of electron accelerators as well as many other applications. It has been developed to the point where reliable tubes produce 50 megawatts peak power and research devices achieve 200 MW at 11.4 GHz for about 10 nanoseconds. T. G. Lee, G. T. Konrad, Y. Okazaki, Masaru Watanabe, and H. Yonezawa, IEEE Trans. Plasma Sci., PS-13, No. 6, 545 (1985), and M. A. Allen et al., LINAC Proc. 508 (1989) CEBAF Report No. 89-001.
Klystrons and gridded tubes provide for most high-power microwave needs. However, they have definite drawbacks for particular applications. Gridded tubes are severely limited in frequency. Power density, gain and efficiency problems rapidly get worse above 100 Mhz. High-power klystrons also have limitations:
they become very large and expensive for the lower frequency range of interest. One solution advanced by Varian Associates is the Klystrode. M. B. Shrader and D. H. Priest, IEEE Trans. Nucl. Sci. NS-32, 2751 (1985); M. B. Shrader, Bull. Am. Phys. Soc. 34, 236 (1989). This device combines some of the features of gridded tubes and klystrons.
For high-power amplifiers, an awkward frequency region exists between approximately 100 MHz and 2 GHz. Moreover, at any frequency, as the peak power increases, designers are forced to use higher voltage to keep the beam current and resulting space charge effects within limits. This means that they are forced to use increasingly relativistic beams which are difficult to axially modulate. In general, it is difficult to achieve high power, high efficiency, high gain, small size/weight, and low cost simultaneously.
Interest has increased in recent years in other methods of microwave generation. A group led by V. Granatstein at the University of Maryland is pursuing the cyclotron maser mechanism for use in a gyroklystron amplifier. Victor L. Granatstein, IEEE Cat. No. 87CH2387-9, 1696 (1987). Another group led by J. Pasour, J. A. Pasour and T. P. Hughes, Bull. Am. Phys. Soc. 34, 185 (1989), is experimenting with the negative mass instability mechanism proposed by Y. Y. Lau, Y. Y. Lau, Phys. Rev. Lett. 53, 395 (1984). Groups at the Stanford Linear Accelerator Center (SLAC), Lawrence Berkeley Laboratory (LBL), and Lawrence Livermore National Laboratory (LLNL) are collaborating on a relativistic klystron project, T. L. Lavine et al., Bull. Am. Phys. Soc. 34, 186 (1989); R. F. Koontz et al., Bull. Am. Phys. Soc. 34, 188 (1989). And recently at Novosibirsk, USSR, where Budker invented the gyrocon, impressive results have been obtained with a version of the gyrocon called the magnicon, M. M. Karliner et al., Nucl. Inst. Meth. A269, 459 (1988).
None of these devices is near commercial production. Further research is required to sort out their relative merits and practical benefits. Reviews by Reid and by Faillon for the accelerator community give summaries of much of the above effort, D. Reid, Proc. 1988 Linac Conf., 514 (1989) CEBAF Report No. 89-001; G. Faillon, IEEE Trans. Nucl. Sci. NS-32, 2945 (1985).